Pyrolysis processes, such as steam cracking, can be utilized for converting saturated hydrocarbons to higher-value products such as C2+ olefin, e.g., ethylene and propylene. Besides these useful products, hydrocarbon pyrolysis can also produce a significant amount of relatively low-value products, such as pyrolysis tar. When the hydrocarbon pyrolysis includes steam cracking, the pyrolysis tar is generally referred to as steam cracker tar (“SCT”).
Pyrolysis tar, including SCT, can be upgraded by conventional hydroprocessing, but doing so leads to reactor fouling and/or catalyst coking leading to a rapid decrease in the amount of upgraded pyrolysis tar that can be recovered. Although catalyst coking can be lessened by operating the process at an elevated hydrogen partial pressure, doing so leads to increased hydrogen demand and equipment costs, which worsen process economics. Catalyst coking can also be lessened by hydroprocessing at relatively low temperatures and diminished space velocity, but these conditions favor undesired hydrogenation reactions.
Although undiluted pyrolysis tar can be hydroprocessed, it is conventional to combine pyrolysis tar with a utility fluid upstream of the hydroprocessing in order to lessen the rate of increase in reactor pressure-drop. Unconverted utility fluid can be separated from the hydroprocessor's effluent, e.g., for recycle and re-use. When no utility fluid is used, a rapid pressure-drop increase is observed across the hydroprocessing reactor, e.g., between the reactor's inlet and outlet. The increased pressure-drop has been attributed to the presence in the SCT of molecules having an atmospheric boiling point ≧565° C., known as “tar heavies”, which include asphaltenes and other high molecular weight molecules.
When the utility fluid has significant aromatics content, the rate of reactor pressure drop is lessened. The hydroprocessed tar product generally has a decreased viscosity, decreased atmospheric boiling point range, and increased hydrogen content over that of the pyrolysis tar feed, resulting in improved compatibility with fuel oil blend-stocks.
U.S. Patent Application Publication No. 2014-0061094 A1 discloses upgrading steam cracked tar in at least one hydroprocessing stage in the presence of a utility fluid. The utility fluid comprises aromatics (i.e., comprises molecules having at least one aromatic core) and has an ASTM D86 10% distillation point ≧60° C. and a 90% distillation point ≦360° C. Even though the rate of pressure drop is lessened, reactor pressure drop eventually reaches a point at which the reactor must be shut down for rejuvenating or replacing the hydroprocessing catalyst and for removing deposits such as coke from the reactor internals.
It is desired to produce such hydroprocessed products in continuous or semi-continuous processes over relatively long time intervals without an undesirable increase in reactor pressure-drop. It particularly desirable to be able to do so for a wide range of utility fluid compositions. Should operation of the pyrolysis tar hydroprocessor and/or associated equipment result in a significant increase in reactor pressure-drop, it is desired to lessen the pressure-drop without a significant interruption of the tar hydroprocessing.